1. Field of the Invention
The present invention relates to preloaded table couplings for shaker test tables.
2. Description of the Prior Art
Shaker tables have long been used for testing various structures, particularly for testing structures that are designed to withstand earthquake loads. The table is moved to simulate an earthquake load by having actuators shake the table in various movements simulating those which occur during an actual earthquake.
There are several problems that have to be solved in adequately moving and loading a table of this type. The motion of the table relative to the reaction mass must be allowed in all degrees of freedom by the load coupling mechanism; the load path must be rigid; and hydraulic power requirements, which can be rather substantial when shaking a large table and the supported test structure must be minimized. Also, the kinematics of the mechanism should be designed to minimize the cross coupling motion between the different axes; the moving mass should be as small as possible to minimize acceleration forces, and keep the natural frequencies high; and the friction of the couplings should be minimized to reduce system acceleration distortion due to friction.
A further factor that has to be considered is that the reaction mass (or foundation mass) is generally made of reinforced concrete. Concrete does not carry tension loads very well, so cyclic loading that involves tension loads complicates the problem of support.
Typical shaking tables may use swivel bearings (part spherical rod end type bearings) on both ends of the actuators that drive the mass, or a separate link may be connected between the load connection and the end of the rod of a rigid pedestal-mounted actuator. When a link is used between the load and actuator, swivel bearings are placed in the ends of the link.
Hydrostatic bearing pairs have been used for driving a table mounted yoke. In such a situation the hydrostatic bearings are mounted on opposite side surfaces of a yoke that attaches to the table. The hydrostatic bearings cannot carry tension, and thus a support that has bearings on its opposite sides is linked to the table. For example, a "U" shaped member with the bottom of the U being parallel to the table, and the legs of the U connected to the table may be used. Hydrostatic bearings are placed on opposite sides of the bottom leg of the "U" in order to provide loading in both directions, (tension and compression) on the side legs of the "U". The bearings react against a toroidal structure which is attached to the actuator rod.
Additionally, in prior art fatigue testing machines used for testing specimens under cyclic loads, the concept of preloading such specimens to a particular level comprising a "mean" load is well known in the art. For example, in U.S. Pat. No. 3,442,120, issued to Russenberger et al. on May 6, 1969, hydraulic cylinders carrying mean loads through a separate loading section of the actuator is shown. The separate section maintains a load on the actuator using accumulators. The reciprocating actuator is cycled in a cyclic load above and below the mean load for fatigue testing. The mean load can be adjusted to desired levels.
The use of hydrostatic slip bearings on table surfaces also is known. These slip bearings may provide a part spherical coupling to the surface, and typical patents showing such slip bearings, which provide a varying pressure in accordance with the load on the bearings and provide a part spherical piston are shown in the U.S. Pat. Nos. 3,994,540 and 3,921,286. Thus, a swivel connection can be utilized through such bearings so long as the bearings are maintained under compression at all times. The bearings will not carry tension loads because the bearing sections will separate.